Static random access memory cell using chalcogenide

ABSTRACT

A static random access memory may be formed using a bitline and a bitline bar coupled to ovonic threshold switches. The ovonic threshold switches may, in turn, be coupled to cross coupled NMOS transistors. In some embodiments, a very compact static random access memory may result.

BACKGROUND

This invention relates generally to semiconductor memories.

One type of volatile semiconductor memory is a static random accessmemory. In the static random access memory, one of two states may bepreserved without the need for refreshing. Static random access memories(SRAM) are highly useful in low power consumption applications,particularly where speed is desirable. Thus, they have found widespreaduse in battery powered applications.

Of course, the cost of the static random access memory is a function ofthe size of its cells. Conventional cells may use four or moretransistors, as well as additional devices such as resistors.

Thus, if the size of the static random access memory could be reduced,the cost of these devices could also be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 1A is a schematic depiction of a cell in accordance with oneembodiment of the present invention;

FIG. 1A is a schematic depiction of a cell in accordance with anotherembodiment of the present invention.

FIG. 2 is a schematic depiction of an array layout in accordance withone embodiment of the present invention;

FIG. 3 is a plot of current versus voltage for an ovonic thresholdswitch in accordance with one embodiment of the present invention;

FIG. 4 is a partial, top plan view of a cell layout in accordance withone embodiment of the present invention;

FIG. 5 is a cross-sectional view taken generally along the line 5-5 inFIG. 4 in accordance with one embodiment of the present invention;

FIG. 6 is a cross-sectional view taken generally along the line 6-6 inFIG. 4 in accordance with one embodiment of the present invention; and

FIG. 7 is a schematic depiction of a system in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a static random access memory (SRAM) cell 10embodiment includes a bitline 18 and an inverse bitline or bitline bar18 a. Connected to each of the bitlines 18 and 18 a is an ovonicthreshold switch 12 a or 12 b. A pair of cross coupled NMOS transistors14 a and 14 b have their gates coupled to one electrode of an ovonicthreshold switch (OTS) 12 a or 12 b. The drains of each transistor 14 aand 14 b are also coupled to the switches 12 a and 12 b. The source ofeach transistor 14 a or 14 b is coupled to a word line 16. Thus, in someembodiments, a two transistor or 2T embodiment may be utilized. However,in other embodiments, other arrangements may be utilized as well.

Thus, referring to FIG. 2, an array may be composed of cells 10, eachcoupled to a pair of bitlines, including a bitline 18 and the bitlinebar 18 b. Each cell 10 is also coupled to a word line 16. Thus, thecells 10 may be arranged in rows constituted by the word lines andcolumns constituted by a pair of bitlines in some embodiments.

Referring to FIG. 3, the operation of an ovonic threshold switch 12 isillustrated. In a low voltage or low field regime “L,” the switch 12 isoff and may exhibit very high resistance in some embodiments. The offresistance can, for example, range from preferably greater than 50,000ohms to be greater than 10 gigaohms at a bias of about half thethreshold voltage. The switch 12 may remain in its off state until avoltage across it exceeds a threshold voltage (V_(TH)) or until acurrent exceeding a threshold current (I_(TH)) switches the device 12 toa highly conductive, low or dynamic resistance “on” state called dV/dIregion that, when extrapolated, will intercept the X axis at the holdingvoltage V_(H).

After turn-on, the voltage across the device 12 drops to a lowervoltage, near the holding voltage V_(H), and remains very close to thisholding voltage almost regardless of the current flowing through theovonic threshold switch since the dynamic on resistance is relativelylow, frequently less than 1000 ohms (in series with the device 12holding voltage).

After passing through the snapback region, in the on state, the device12 voltage drop remains close to the holding voltage as the currentpassing through the device is increased, even if at a relatively high,read or write current level. Above a relatively high current level(density) the device remains on but displays a finite and increasingdynamic resistance, with the voltage drop across 12 a increasing withincreasing current due to the IR drop across the dynamic resistance.

The device 12 may remain on until the current through the device 12 isreduced below a characteristic holding current value that is dependenton the type and area of the material, and may be impacted by the top andbottom electrodes utilized to form the device 12, as well as themagnitude of capacitance on the column line.

In some embodiments of the present invention, the device 12 does notchange phase. It remains permanently amorphous and its current-voltagecharacteristics may remain the same throughout its operating life.

As an example, for a 0.5 micrometer diameter device 12, with a thicknessof approximately 750 Angstroms, formed of TeAsGeSSe having respectiveatomic percents of 16/13/15/1/55, the holding current may be on theorder of 0.1 to 1 micro-amps (uA) in one embodiment. Below this holdingcurrent, the device 12 turns off and returns to the high resistanceregime at low voltage, low field in FIG. 3.

The threshold current for the device 12 may generally be of the sameorder as the holding current, or preferably greater to avoid oscillationif driven by a high impedance current source. The holding current may bealtered by changing process variables, such as the top and bottomelectrode material and the chalcogenide material. The device 12 mayprovide high “on current” for a given area of device compared toconventional access devices such as metal oxide semiconductor fieldeffect transistors or bipolar junction transistors or semiconductordiodes.

In accordance with one embodiment of the present invention, a voltage Vmay be chosen as the maximum operating voltage for the memory array. Inone embodiment, it may be 3 volts. The OTS devices 12 may be providedwith a threshold voltage, V_(TH), between 2V/3 and V in someembodiments. The holding voltage V_(H) of the OTS devices 12 may bebetween V/3 and 2V/3 in some embodiments. All unselected word lines maybe biased to V/3 in some embodiments. All unselected bitlines andbitline bars may be biased to 2V/3 in some embodiments. Then, selectedword lines 16 may be biased at a voltage less than the unselected wordline voltage.

For reading, the selected bitlines and bitline bars may be ramped from2V/3 towards V using a current source or, as another example, they maybe pre-charged at V, and then one of the bitlines may be discharged whencurrent is forced through a selected cell's word line, such as when theword line is selected by lowering it.

For writing, the selected cell's word line may be biased at 0 volts forone embodiment. To write a 1, the selected bitline 18 is biased at V,while the selected bitline bar 18 a is kept at 2V/3. To write a 0, theselected bitline bar 18 a is biased at V, while the selected bitline 18is kept at 2V/3.

The cross coupled NMOS transistors 14 may have threshold voltages lessthan V/3. The threshold voltage of the transistors 14 may be higher toassure that the sub-threshold current into the drain of the offtransistor is substantially less than the positive pull-up current fromthe OTS device 12 on the drain of the off transistor over the specifiedtemperature range. That is, the leakage current of an OTS device 12 withless than 0.1 volts across the OTS 12 may be more than the leakage ofthe off transistor 14. The substrate may be biased or pumped negativerelative to the most negative voltage imposed on the word line 16 toavoid unreasonably high body effect that can lower the threshold voltagewhen the word line is low and cause excessive sub-threshold transistorleakage in off transistors, in some embodiments.

In the following discussion, a detailed embodiment of a write and readoperation is provided. However, the present invention is not limited toany particular read or write operation, and these examples are givenwithout limiting the scope of present invention.

In such an embodiment, all of the word lines 16 may initially be biasedto V/3. Then, the bitlines 18 and bit line bars 18 a are biased to 2V/3.If the cell was previously in the 0 state, the NMOS transistor 14 aremains on and the node A follows, being pulled down to 0 volts fromsource to drain, so both source and drain are at about V/3. With theNMOS transistor 12 a on, there is a high capacitive coupling between thegate of the NMOS transistor 12 a, its drain, channel, and source. Thus,the node B is coupled down from 2V/3 towards V/3 if the word line 16 islowered to 0 volts, such as during a write cycle.

To write a 1, the bitline 18 is brought to V, while the bitline bar 18 aremains at 2V/3. The OTS 12 a now has V volts applied across it. Thisvoltage is greater than its threshold voltage, turning on the device bygoing to a low resistance in series with the holding voltage, which maybe between V/3 and 2V/3. The OTS 12 b has close to V/3 applied across itand remains off while 12 a turns on, since the threshold voltage of thedevice 12 b is also greater than V/3. The OTS 12 a is on and has a verylow on resistance relative to the source/drain resistance of the ontransistor 12 a.

Thus, the cross coupled latch, formed of the transistors 14, switchesstates by turning on NMOS transistor 12 b with a low source impedancegate voltage and this turns off NMOS transistor 12 a, due to node Bbeing pulled down to 0 volts when transistor 12 b turns on, as its gatevoltage is raised by the OTS 12 a.

Node A is then driven to V minus the holding voltage of the OTS 12 whichis greater than V/3 because the holding voltage of the OTS 12 is lessthan 2V/3 and the word line 16 is still at 0 volts.

Next, the bitline 18 is reduced from V to 2V/3. With the NMOS transistor14 b now on, there is high capacitive coupling between the gate of theNMOS transistor 14 b and its drain, channel, and source. Thus, when thebitline 18 is lowered from V to 2V/3, node A starts at 2V/3 or less (ifV_(H) is greater than V/3). However, node A declines only slightly sincethe OTS device 12 a turns off promptly as the selected bitline at V islowered, and node A remains at a value greater than V/3 and less than Vminus the holding voltage of the OTS device 12, since the node A startedat V minus the holding voltage of the OTS and the capacitance on node Areduces the voltage across the OTS device as the bitline is lowered(instead of lowering node A as the bitline is lowered.

A capacitive divider action after an OTS device turns off lowers node Afrom V minus the holding voltage of the OTS device 12 a as the bitline18 voltage lowers. The capacitance across the OTS device 12 a may,preferably, be much less than the transistor and parasitic capacitanceon node A. The holding voltage of the OTS may, preferably, be adjustedto be enough less than 2V/3 to accommodate the capacitive coupling downof node A from lowering the bitline 18 to 2V/3 so that node A remainsmore than V/3 greater than the word line 16 potential after bitline 18returns to 2V/3, upon deselection after write. If node A remains morethan V/3 greater than the word line 16 potential and V_(TH) (transistors14) are less than V/3, then transistor 14 b is assured to remain on.

Because node A is pulled up to and starts at V minus the holding voltageof the OTS during the write cycle, the voltage across the OTS device 12a falls below its holding voltage and turns off when the bitline islowered from V to 2V/3 since the voltage across the OTS device 12 a isreduced from its holding voltage by V/3, since node A remains relativelyconstant as the bitline 18 is lowered (due to capacitance mostly fromnode A through “on” transistor 14 b).

Lowering the bitline to 2V/3 leaves the node A at more than 2V/3 minusthe holding voltage of the OTS, because of the capacitive coupling ofthe node A to the bitline Preferably, node A is not less than V/3, byassuring that the holding voltage of the OTS is adequately less than2V/3 to accommodate capacitive coupling. Similarly, the holding voltageof the OTS may be less than 2V/3 so that when the word line voltagerises, the voltage from the word line 16 to node A stays greater thanV/3. Here, the capacitive coupling as the word line is deselected backto V/3 reduces margin (voltage between node A and the word line thatkeeps transistor 14 b on). Accordingly, the capacitive ratio, V_(H), andV_(TH) are preferably adjusted so that transistor 14 b remains on whenthe word line is deselected, such as to V/3. Even if transistor 14 bturns off, node B is lower than node A, so node A rises enough to turntransistor 14 b back on before node B rises to turn transistor 14 a on(after deselection).

After the word line 16 is brought back up to V/3, because of the NMOStransistor 14 b, there is high capacitive coupling between the gate ofthe NMOS transistor 14 b and its drain, channel and source. Thus, node Acouples up to a value preferably greater than 2V/3 and less than V. Vand the holding voltage of the OTS are adjusted for parasiticcapacitance ratio to prevent the voltage between node A and the wordline 16 from being less than a threshold voltage plus an on voltage,where the on voltage is adequate to maintain the cell state during wordline switching from V/3 towards 0 during reading. If transistor 14 bremains on when the word line 16 returns to V/3, node B is also broughtup to V/3, following and remaining about equal to the word linepotential at V/3 because transistor 14 b remains on during thetransitions of the word line during standby and read. Should margindecay and transistor 14 b turn off as word line 16 is deselected, thenode B may temporarily remain less than V/3 and gradually be charged upto V/3 by OTS device 12 b (or pulled up once transistor 14 b turns afternode A is charged up by device 12 a to be above word line 16 plus thethreshold voltage of transistor 14 b.

To write a 0 into node A where the cell is previously in the 0 state andhas a low voltage on node A relative to node B, the word line of theselected cell is pulled down to 0 volts. The NMOS transistor 14 aremains on and node A follows, being pulled down to 0 volts. With theNMOS transistor 14 a on, there is a high capacitive coupling between thegate of the NMOS transistor 14 a and its drain, channel, and source.Thus, node B is coupled down from 2V/3 to close to V/3 when the wordline voltage goes down.

To write 0, the bitline bar 18 a is brought up to V, while the bitline18 remains at 2V/3. The OTS device 12 a now has 2V/3 applied across itand remains off. The OTS device 12 b has 2V/3 applied across it and italso remains off. Thus, neither OTS device 12 turns on.

Next, the bitline bar 18 a is brought back to 2V/3. With NMOS transistor12 a on, there is high capacitive coupling between the gate of the NMOStransistor 14 a and its drain, channel, and source. Thus, node B remainsat a value close to V/3, the voltage it was at before the bitline bar 18a was raised to V. Then, the word line 16 is brought back up to V/3.Again, with the NMOS transistor 14 a on, there is a high capacitivecoupling between the gate of the NMOS transistor 14 a and its drain,channel, and source. Thus, node B couples up to 2V/3, keeping the NMOStransistor 14 a on and node A is also brought up to V/3, following theword line potential.

For the case where the cell is previously in the one state, writing a 1or a 0 is symmetrically analogous to the procedure described above.

A worse case write condition may exist where the on voltage for anunselected bit may be increased by writing another bit repeatedly on thesame column. In this case, a bit is repeatedly written on the samecolumn to the same state on the bitline. For this repeating write cycle,a bitline is repeatedly pulled to V from 2V/3 for write and then back upto 2V/3 after write. For example, a bitline spending half of the time atV, compared to 2V/3, will be at an average voltage of 2.5V/3, thevoltage seen by other bits on the column that are not written. Sincethis is the effective pull-up voltage on the column through which theOTS device charges node A (or node B if it is bitline bar that is notactively driven low by an on transistor. Thus, the OTS device 12 pullsthe “off” node up to the average voltage on the bit line, and internalnode A on the unwritten bits is pulled to 2.5V/3 because the internalnode impedance is greater than the time of a write cycle.

After the column is “hammered” so long that the internal node is as highas it can go on the unwritten bits, the opposite state is written to acell that is on the hammered column, but was not the cell selectedduring the hammering. Here, the drive on the on transistor 14 b isgreater due to the higher gate voltage, so that more current is forcedinto node B through the bitline bar when bitline bar is the bit linepulled to V. To minimize the increase in worse case current necessarythrough the OTS device 12, the duty cycle may be minimized and,preferably, does not exceed a given percentage, such as 50 percent.Otherwise, V may be increased or the maximum threshold voltage reducedso that the ratio of the on voltage (the voltage on node A minus thethreshold voltage minus the word line voltage) for its maximum valueversus its minimum value during write is reduced to assure thatexcessive current is not required through the OTS device to overcome theon resistance. Similarly, the minimum on voltage may be chosen byincreasing V and/or reducing threshold voltage so that when the wordline is pulled to ground, the transistor is on hard enough so that theon transistor keeps the node reasonably near the word line voltage toavoid the possibility of flipping the bit if there is a mismatch betweenthe internal nodes A or B of a given bit.

Also, V_(H) and V may be adjusted so that the minimum on voltage(Vonmin) is maintained and adequate even if there is significantparasitic capacitance on node A or feedthrough capacitance from the OTSdevice. That is, to increase Vonmin, V_(H) may be reduced and/or Vincreased to minimize drain-source “on” voltage for the “on” transistor,when the word line is lowered so that the opposite transistor in thecell does not tend to come on and “flip” the bit—causing a bit disturbcondition.

For cases with less margin, where a bit may have just been written andthe node B (driven by the off transistor) is not yet high enough whenthe word line is deselected to V/3, read or write disturb may beavoided. To minimize the source to drain voltage for the on transistorduring the transition of the word line from V/3 to 0, the word line maybe driven with a controlled, relatively slow edge rate initially, suchas with a current source, for at least 50% or more of the transition sothe transistor is turned on with more Von before driving the word lineat a faster dV/dt rate to ground.

A detailed example of a read operation is now provided without limitingthe scope of the present invention. Initially, such as in standby ordeselect or precharge, all word lines are biased at V/3. All bitlinesand bitline bars are biased at 2V/3. Consider the case where the cell ispreviously in the 0 state. For this case, the NMOS transistor 12 a isturned on with negligible drain to source voltage drop. Thus, the node Ais at a potential of V/3. Transistor 12 b is turned off. The off stateresistance of the NMOS transistor 12 b is preferably much higher thanthat of the OTS device, so that most of the bias voltage between bitline and word line is dropped across the off transistor instead of theoff OTS device 12. Thus, node B is at a potential close to 2V/3 beforethe word line potential is lowered.

To initiate the read cycle, the word line of the selected cell is pulledlower than the deselect voltage, such as from V/3 to 0V. NMOS transistor12 a remains on and node A follows, being pulled down to 0V because nodeA is actively driven by an on transistor 12 a. With NMOS transistor 12 abeing on, there is high capacitive coupling between the gate of NMOStransistor 12 a and its drain, channel, and source. Thus, node B iscoupled down from about 2V/3 to close to V/3 when the word line islowered because the transistor feed-through capacitance from gate to thesource/drain is most of the capacitance on node B.

During the pulldown on the word line, the bitlines were pre-charged andequalized at 2V/3. Next the pre-charge is released, and a current sourcemay be applied to both the bitline and bitline bar, causing the selectedbitline and bitline bar voltages to rise at a linear rate until the OTSdevice, driven by the on transistor 12 a, triggers on, thus dis-chargingthe bitline and its array capacitance on one of the columns being read,such as bitline 18 if transistor 14 a was on and node A lower than nodeB at the start of the read cycle. Since node A is at 0V and activelydriven there, while node B is at V/3 and basically floating, OTS device12 a turns on first. This pulls down the bitline voltage to V (wordline)+V_(H) (OTS), or about 0V plus V_(H), or between V/3 and 2V/3.Bitline bar will charge towards V and above, but may be clamped beforeit reaches V to assure bitline bar remains less than V. If V_(H) isclose to 2/3V, then the bitline clamps at or below 2V/3. If V_(H) isclose to V/3, then the bitline clamps at V/3. Bitline bar still chargestowards V regardless of the voltage on node A, so long as the differencebetween node A and word line 16 remains less than V_(TH).

The transient current through 14 a increases the source-drain voltagedrop of on transistor 14 a. If this voltage exceeds V_(TH), thentransistor 14 b turns on (instead of remaining off as desired). Tobetter avoid that condition, the capacitance on bitline 18 may beminimized for improved margin. For example, the capacitance on bitline18 may be less than the capacitance on node A so the capacitive dividerdoes not force a source-drain voltage across transistor 14 a thatexceeds the threshold voltage of the transistor 14 b. Or the onresistance of device 12 a may be increases to be greater than the worstcase Ron(14 a) to better assure the transient source-drain on voltagedoes not exceed the threshold voltage of the transistor 14 b.

Since V_(H) is less than 2V/3, a sense amplifier can sense thedifferential between bitline and bitline bar and amplify the resultrepresenting the data stored in the cell because bitline may be rapidlydriven down to less than 2V/3 when OTS 12 a turns on, whereas theoff-side bitline bar rises above 2V/3 even if OTS 12 b comes on sincetransistor 12 b is off. OTS 12 b may come on, for example, if thethreshold voltage of the OTS is closer to 2V/3 instead of V, and if thegate voltage is less than V/3 (though node B needs to stay greater thanVth (transistor 12 b)+Von, to avoid transistor 12 a turning off).However, even with OTS 12 b triggering on and then pulling node B towithin V_(H) of the bitline bar voltage (since presumably the columncapacitance is much greater than node B capacitance), the bitline barcan continue to charge up since transistor 14 b is off, so node B andits capacitance and load can be driven high rapidly, thus, increasingthe turn-on to Von above threshold voltage (Vgate-source) for transistor14 a.

After latching the difference on the bitline and bitline bar columns,the bitline and bitline bar current sources may be turned off. The wordline may be raised to V/3 while the bit lines are forced to 2V/3. Thisturns off OTS 12 a by assuring V/3 or less is across the OTS 12 a, sincenode A is actively driven to remain equal the word line. The state ofthe cross-coupled NMOS transistors 14 is undisturbed and the readoperation is non-destructive.

With NMOS transistor 12 a being on, there is high capacitive couplingbetween the gate of NMOS transistor 12 a and its drain, channel, andsource. Thus, node B remains at or above V/3 above the word line beforeand after the word line is lowered to 0 and raised back to V/3. The RC(through the R of the untriggered (OFF) OTS into the gate capacitance(node B) of the on transistor) may be high enough so that the internalnode B is only gradually driven through the OFF OTS to be different fromthe capacitively coupled voltages on the gate of the on transistor(drain of the off transistor).

Thus, because of this capacitive coupling, node B couples up to avoltage above V/3, and preferably above V/3 plus the threshold voltageof transistor 14 a, when the word line is raised from 0V to 1/3V. If OTS12 b came on during the read cycle (because the V_(H) is closer to V/3),the node B could be at 2V/3 or greater before the word line is raisedfrom 0 to V/3. Then, when word line is raised from 0 to V/3, the node Bcould couple from 2V/3 to V or higher. Still, this assures that morethan V/3 remains across the gate to source of the on transistor 12 a.This keeps NMOS transistor 12 a on and node A is also brought up to V/3,following the word line potential as the word line is raised from 0Vback to V/3.

If V_(H) is much less than 2V/3 (by being closer to V/3), the “on”column during read can be less than 2V/3 for a significant percentage oftime when the word line is at 0V—depending on duty cycle. This maydegrade the average Von voltage for other deselected bits in standby onthat column. This is because the word line for other deselected bitsremains at V/3, but the column bitline or bitline bar voltage for thebit being read can be less than 2V/3, the column bitline voltageconnected to other deselected bits.

A repeating read may be run on a selected bit or bits in the selectedcolumn where the selected column is repeatedly read with the samebitline being the low or “ON” side. If the duty cycle is 50% or greater,the selected column averages V/2 or even less in voltage, depending onV_(H). Accordingly, assuring a low duty cycle, preferably less than 25%,minimizes the degradation in Von for deselected bits in someembodiments. Making V_(H) at or near the minimum threshold voltage ofthe OTS devices so that V_(H) is preferably closer to 2V/3 instead ofV/3, also minimizes degradation in Von for bits in standby duringrepeated reads.

Alternatively, the selected column bit lines may be precharged andequalized to V before the word line is selected. Then, by pulling theword line down with a current source with a controlled slope (dV/dT),the OTS in the selected cell that is connected to the lowest drainvoltage turns on first because more voltage is across the OTS. Thetransistor 12 a turns on the OTS 12 a driven by transistor 12 a, andthen drives the bitline low while the other bitline remains relativelyhigh, since it is charged low only by leakage. This approach keeps thelower bitline mostly above 2V/3 and avoids degrading the high node instandby cells.

Other alternatives that improve performance and margin will be apparentto those reasonably skilled in the art. For example, if the bit linesare equalized and charged high or low, the rate of change in the columnmay be sensed. When the rate of charging changes, one of the OTS device12 has turned. Accordingly, the read currents charging the two selectedbitlines 18 and 18 a may both be turned off, the voltage levels comparedto determine cell state from which is higher, and the levels pre-chargedback to the deselect level. This approach may minimize delay andminimize any changes for average deselect bit levels during read.

Similarly, during write, the current into the column selected may besensed as the column is driven from 2V/3 to V. If the current does notincrease after a brief period (e.g., 10 nsec) or if it increases andthen decreases, the write cycle may be terminated. This similarlyminimizes the write cycle time and decreases the time the bit lines areat other than the deselect voltage. To ease the write current, the Ronof the transistors 14 may be increased (by adjusting the width or lengthof transistors 14). If Ron (transistor) is less then Rdyn (OTS 12), thecell will write more readily at less current. Similarly, if Rdyn (OTS12) is low so the drop at peak write current is low, the voltage marginmay improve.

As a further non-volatile alternative, the 12 may be composed of OTSdevices 12 a, 12 b and phase change memory cells (OUM) 13 a, 13 b asindicated in FIG. 1A. For example, an OUM device 13 may be in serieswith each OTS device 12. The OUM may be formed of Ge₂Se₂Te₅, in oneembodiment. Then, the OUM may be written to a low or high resistancetechniques using the techniques described herein. Further, theresistance may be decreased by terminating the write cycle with a slowtrailing edge (such as slowly restoring the bit line and/or word line tothe deselect level, such as with an edge rate greater than 500 nsec).Similarly, the resistance may be increased by terminating the writecycle with a fast trailing edge (such as by restoring the bit lineand/or word line to the deselect level with a fast edge rate, such asfaster than 10 nsec).

Where an OUM is used alone, the use of fast edge rates maintain the OUMin a relatively high resistance state. Similarly, where an OUM is usedin series with an OTS, the OUM may simply add to the resistance of theOTS by terminating cycles with a slow edge rate. However, writing theOUM 13 a, 13 b associated with both OTS devices 12 a and 12 b to theappropriate state so the state is retained on power off, each may besuccessively “written”—one with a slow terminating edge rate and theother with a fast terminating edge rate.

For high performance operation, the OUM portion of device 12 may bemaintained in the low or lower resistance state during normal operationand written on power-down, so the cell state is retained on power up.Alternatively, the OUM may be maintained in the high resistance(threshold state) during normal operation (through use of fast edgerates during read and write), and then one side of the cell written tolow resistance state on power down.

FIGS. 4-6 show an example of a layout for the OTS SRAM cell 10. The twoNMOS transistors 14 a and 14 b are cross-coupled using the OTS lowerelectrode L-shaped conductor 40. For the OTS devices 12 a and 12 b,their top terminal electrodes are integral with bitlines 18 or bitlinebars 18 a respectively. The word line 16 spans from left to right,making contact in the middle of the active area A between thecross-coupled transistors 14 a and 14 b.

In this example, metal 2 is used for the word lines 16 and metal 1 isused for the bitlines and bitline bars 18, 18 a. The OTS lowerelectrode, L-shaped conductor 40 is located below the metal 1 level. Thecontact down to active area A from the word line 16 may be done using anintermediate plug 36 b from metal 2, forming the word line 16, down tometal 1 contact 18 b. Then another plug 36 a may extend from the metal 1contact 18 b down to the active area A to contact the common source 30 bof the transistors 14 a and 14 b. Thus, looking at FIG. 4, the topmostlevel is the level metal 2, which forms the word line 16. Below metal 2are plugs 36 a used to contact from metal 2 down to metal 1. Under thecontact plugs 36 a is the metal 1 which forms the bitlines 18 and thebitline bars 18 a. A contact plug 18 b under metal 1 contacts thesilicon active area A.

The drain 30 a of each transistor 14 a, 14 b is coupled by plug 30 c tothe L-shaped conductor 40, which also forms the lower conductor of anOTS 12 a or 12 b, and also cross-couples the respective transistor drain30 a to the other transistor's gate 34 as further indicated in FIG. 6.Thus, the OTS 12 a or 12 b includes an upper conductor, in common withthe bitline 18 or bitline bar 18 a, the chalcogenide 38, and theconductor 40, that also forms the interconnection for cross-coupling thedrains 30 a of each transistor to the other transistor's gate. Thus, thecross coupling of the gates 34 to the drains 30 a may be achieved by theL-shaped conductor 40 and the plug 31 that extends downwardly to contacta gate 34 on one end and couples an OTS 12 on the other end by formingthe OTS's lower electrode. The gates 34 may be polysilicon in oneembodiment. The areas between conductive lines are insulating ordielectric layers (not shown).

With very low temperature processing after depositing the chalcogenide38, alloy 1938 may be used as the chalcogenide 38. For highertemperature processing after chalcogenide deposition and forapplications allowing higher leakage levels and standby current, “stag”alloy 1040 may be used as the chalcogenide 38. TiSiN or TiAlN may beused between aluminum or copper interconnect layers, for example, forthe conductor 40. Tungsten (W) or Aluminum (Al) may be used for thebitline 18. The plugs 30, 39, 36 a, and 36 b may be formed of tungstenin one embodiment.

The SRAM cell 10 may be embedded in combination with a non-volatilememory cell. For example, in non-volatile memory in the form of a phasechange memory with an OTS select device, the OTS devices may befabricated to act as select device for the phase change memory and alsoserve as part of SRAMs cells as outlined herein. This allows theaddition of a compact more efficient SRAM memory cell with lessadditional processing. Further, this approach can beneficially add a lowcost cache buffer to the non-volatile Flash that uses a phase changememory. The SRAM cell can be placed underneath a phase change memoryarray since the phase change memory array does not need an activeisolation device within or under the memory cell. The SRAM cell 10 maybe written by the non-volatile cell (such as at power-up or upon commandby the external user through an input/output interface). Similarly, theSRAM cell 10 may be loaded into the non-volatile memory upon poweringdown.

In some embodiments, OTS SRAM cells may be much smaller than 4T or 6TSRAM cells. Size reduction reduces the cost of the SRAM memory, whetheras applied to stand alone commodity memory or as embedded memory on achip that provides other functionality, such as a microprocessor ordigital signal processor. It also provides for potential increasedmemory capacity while keeping chip size within reasonable limits asdictated by package size and die yield constraints.

In comparison with 4T SRAM cells, the vertical height of the cell can bereduced due to the relatively short height of the OTS device as comparedto vertical intrinsic resistors, which have to be tall enough to avoidvoltage breakdown at max supply voltage. The area of the cell can bereduced since the area of the pass gates connected to the word line maybe eliminated as well as the connection to ground (saving one metal lineor layer).

Turning to FIG. 7, a portion of a system 500 in accordance with anembodiment of the present invention is described. System 500 may be usedin wireless devices such as, for example, a personal digital assistant(PDA), a laptop or portable computer with wireless capability, a webtablet, a wireless telephone, a pager, an instant messaging device, adigital music player, a digital camera, or other devices that may beadapted to transmit and/or receive information wirelessly. System 500may be used in any of the following systems: a wireless local areanetwork (WLAN) system, a wireless personal area network (WPAN) system,or a cellular network, although the scope of the present invention isnot limited in this respect.

System 500 may include a controller 510, an input/output (I/O) device520 (e.g. a keypad, display), a memory 530, and a wireless interface540, coupled to each other via a bus 550. A battery 580 may supply powerto the system 500 in one embodiment. It should be noted that the scopeof the present invention is not limited to embodiments having any or allof these components.

Controller 510 may comprise, for example, one or more microprocessors,digital signal processors, microcontrollers, or the like. Memory 530 maybe used to store messages transmitted to or by system 500. Memory 530may also optionally be used to store instructions that are executed bycontroller 510 during the operation of system 500, and may be used tostore user data. The instructions may be stored as digital informationand the user data, as disclosed herein, may be stored in one section ofthe memory as digital data and in another section as analog memory. Asanother example, a given section at one time may be labeled as such andstore digital information, and then later may be relabeled andreconfigured to store analog information. Memory 530 may be provided byone or more different types of memory. For example, memory 530 maycomprise a volatile memory (any type of random access memory), anon-volatile memory such as a flash memory, a phase change memory,and/or the static random access memory 10 illustrated in FIG. 1. Thestatic random access memory described herein may communicate with theother elements of system 500 or with the external user through the I/Obus or otherwise (such as wireless) by techniques that are familiar tothose reasonably skilled in the art.

The I/O device 520 may be used to generate a message. The system 500 mayuse the wireless interface 540 to transmit and receive messages to andfrom a wireless communication network with a radio frequency (RF)signal. Examples of the wireless interface 540 may include an antenna,or a wireless transceiver, such as a dipole antenna, although the scopeof the present invention is not limited in this respect. Also, the I/Odevice 520 may deliver a voltage reflecting what is stored as either adigital output (if digital information was stored), or it may be analoginformation (if analog information was stored).

While an example in a wireless application is provided above,embodiments of the present invention may also be used in non-wirelessapplications as well.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A static random access memory comprising: a static random accessmemory cell including a chalcogenide that is always in an amorphousphase and cross coupled NMOS transistors, wherein the gate of eachtransistor is coupled to the drain of the other transistor, said memorycell including first and second ovonic threshold switches, saidchalcogenide being part of said switches.
 2. The memory of claim 1including an ovonic threshold switch including said chalcogenide.
 3. Thememory of claim 2 including a phase change memory in series with saidswitch.
 4. The memory of claim 1 wherein said transistors have a commonsource.
 5. The memory of claim 1 wherein one transistor's gate iscoupled to said first ovonic threshold switch and the other transistor'sdrain is coupled to said first ovonic threshold switch.
 6. The memory ofclaim 5 including an L-shaped metal layer having opposed ends, saidlayer coupled to one said NMOS transistor drain on one end, a regionbetween the ends forming the lower conductor of said first ovonicthreshold switch and said layer coupled to a gate electrode of the othersaid NMOS transistors on the other end.
 7. The memory of claim 6including a plug from said layer to said gate electrode.
 8. The memoryof claim 6 including a bitline coupled to said first ovonic thresholdswitch and a bitline bar coupled to said second ovonic threshold switch.9. A method comprising: forming a static random access memory using achalcogenide that is always in the amorphous phase; forming said staticrandom access memory using cross coupled NMOS transistors; coupling thegate of each transistor to the drain of the other transistor; andforming a first and second ovonic threshold switch and coupling the gateof one transistor to said first ovonic threshold switch and the drain ofthe other transistor to said first ovonic threshold switch.
 10. Themethod of claim 9 including forming an ovonic threshold switch usingsaid chalcogenide.
 11. The method of claim 10 including forming a phasechange memory in series with said ovonic threshold switch.
 12. Themethod of claim 11 wherein a phase change memory state is determined bya trailing edge of a read or write termination cycle.
 13. The method ofclaim 9 including forming a common source for said NMOS transistors. 14.The method of claim 9 including forming an L-shaped metal layer havingopposed ends such that one end of said layer is coupled to one said NMOStransistor drain, a region between the ends forms a lower electrode ofsaid first ovonic threshold switch and the other end of said layer iscoupled to the gate electrode of the other said NMOS transistors. 15.The method of claim 14 including forming a plug from said layer to saidgate electrode.
 16. The method of claim 14 including coupling said firstovonic threshold switch to a first bitline and said second ovonicthreshold switch to a second bitline.
 17. The method of claim 9including reading said memory by charging a pair of bitlines and thendischarging one of the bitlines relative to the other.
 18. The method ofclaim 9 including writing said memory by biasing one of said bitlinescoupled to said cell higher than the other of said bitlines.